In a work vehicle such as a bulldozer, engine output is transmitted to a drive wheel (sprocket) via a transmission (hydraulic clutch). The transmission is comprised of a forward travel gear selected by connection of a forward clutch, a reverse travel gear selected by connection of a reverse clutch, and speed gears selected by switching connection of speed-change clutches.
In a work vehicle, a forward operation command is output when an operator operates a travel operation device such as an operation lever to a forward travel direction position (“F” position). When the forward operation command is output, the forward clutch is connected, and engine power is transmitted to the drive wheel via the forward travel gear and a currently selected speed gear of the transmission. This causes the vehicle to travel forward. When the operator operates the travel operation device to a reverse travel direction position (“R” position), a reverse operation command is output. When the reverse operation command is output, the reverse clutch is connected, and engine power is transmitted to the drive wheel via the reverse travel gear and a currently selected speed gear of the transmission. This causes the vehicle to travel in reverse.
When the forward motion of the work vehicle is switched to the reverse motion or vice versa, it is generally preferable for protection of the clutch to halt the work vehicle before operating the travel operation device to switch between forward and reverse operation.
However, it is very much a situation in which the travel operation device is, for example, operated to start reverse motion while the vehicle is still traveling forward, for the purpose of shortening the cycle time.
When the operator operates the travel operation device and a reverse operation command is output while the work vehicle is traveling forward, the reverse clutch is connected gradually. This causes the vehicle's kinetic energy to be absorbed by the reverse clutch, and the vehicle is decelerated.
When the vehicle, which is traveling forward, is decelerated by the connection operation of the reverse clutch, problems as follows will be incurred.
(A) When the reverse clutch is set to the slip state, such power as to cause the vehicle to travel in reverse is momentarily transmitted from the transmission to the drive wheel. This produces a transmission shock.
(B) The reverse clutch is subjected to a significant thermal load in order to decelerate the vehicle. This reduces the lifetime of the reverse clutch. It is conceivable that the reverse clutch connection operation is performed in a short period of time in order to reduce the thermal load applied to the reverse clutch. In this case, however, the vehicle will be decelerated abruptly, causing significant transmission shock.
(C) The reverse clutch connection operation is in need of being performed in a short period of time when the vehicle is caused to travel in reverse after deceleration operation in order to prevent the reverse clutch which has been subjected to a substantial thermal load from being subjected to more thermal load. Such connection operation will also cause a transmission shock.
Similar problems will also occur when the vehicle which is traveling in reverse is switched to the forward travel.
(Related Art 1)
Patent Document 1 describes an invention in which when an operation lever is switched from the forward travel direction position to the reverse travel direction position, both a forward clutch and a reverse clutch are set to the disconnected state while a speed-change clutch is connected appropriately, so that the vehicle's kinetic energy is absorbed by the speed-change clutch to decelerate the vehicle. The same applies to when switching from the reverse travel direction position to the forward travel direction position.
(Related Art 2)
Patent Document 2 describes an invention in which when an operation lever is operated to switch from the forward travel direction position to the reverse travel direction position, the clutch connection is switched from a forward clutch to a reverse clutch while automatically activating a brake if the vehicle speed is higher than a certain vehicle speed. The same applies to when switching from the reverse travel direction position to the forward travel direction position.
(Related Art 3)
Patent Document 3 describes an invention in which when an operation lever is operated to switch from the forward travel direction position to the reverse travel direction position, the clutch connection is switched from the forward clutch to the reverse clutch while activating a brake if the vehicle speed is higher than a certain vehicle speed, whereas if the vehicle speed is the certain vehicle speed or lower, the engagement force between the forward clutch and the reverse clutch is controlled. The same applies to when switching from the reverse travel direction position to the forward travel direction position.
(Related Art 4)
Patent Document 4 describes an invention in which when an operation lever is operated to switch from the forward travel direction position to the reverse travel direction position, the clutch connection is switched from the forward clutch to the reverse clutch while activating a brake so as to increase the braking force if the vehicle speed before the switching operation is high, whereas if the vehicle speed before the switching operation is low, the clutch connection is switched from the forward clutch to the reverse clutch while activating the brake so as to reduce the braking force. The same applies to when switching from the reverse travel direction position to the forward travel direction position.    [Patent Document 1] Japanese Patent Application Laid-Open No. 06-272758    [Patent Document 2] Japanese Patent Application Laid-Open No. 03-128730    [Patent Document 3] Japanese Patent Application Laid-Open No. 06-92162    [Patent Document 4] Japanese Patent No. 2680479